Cellulose esters are polymers, which are useful in many plastic, film, coating and fiber applications. In coatings applications, cellulose esters are generally applied from organic solvent solutions. However, in an increasing number of industries, aqueous coating compositions continue to replace traditional organic solvent-based coating compositions. Paints, inks, sealants, and adhesives, for example, previously formulated with organic solvents are now formulated as aqueous compositions. This reduces potentially harmful exposure to volatile organic compounds (VOC's) commonly found in organic solvent-based compositions. While the move from organic solvent-based to aqueous compositions brings health and safety benefits, the aqueous coating compositions must meet or exceed the performance standards expected from solvent-based compositions. The need to meet or exceed such performance standards places a premium on the characteristics and properties of such aqueous coating compositions.
U.S. Pat. Nos. 5,668,273 and 5,994,530 disclose the use of carboxylalkyl cellulose esters, as well as their use in aqueous pigment dispersions and coating compositions. U.S. Pat. No. 5,668,273 discloses various new ether derivatives of cellulose esters, which are described as being useful in coating compositions as binder resins and rheology modifiers. These esters are described as having good solubility in a wide range of organic solvents, compatibility with a variety of resins, and ease of dispersion in a water-borne formulation. U.S. Pat. No. 5,994,530 describe carboxymethyl cellulose esters of higher acids that are used in aqueous dispersions, which are useful in formulating waterborne coating compositions containing pigments.
Methods of preparing carboxylalkyl cellulose esters are known in the art. For example, U.S. Pat. Nos. 5,973,139; 5,792,856; and 4,590,265 describes carboxylated cellulose esters, and various synthetic processes for their preparation. U.S. Pat. No. 5,973,139 describes a process which includes activating an oxidized cellulose with water, followed by dehydration using acetic acid, and optionally displacing at least some of the acetic acid with butyric acid or propionic acid. The activated cellulose ester is then reacted with an esterifying reagent in the presence of a catalyst. The temperature is gradually increased to a level sufficient to complete the esterification. The carboxylated ester is then hydrolyzed to obtain the desired degree of substitution, and precipitated from solution.
U.S. Pat. No. 5,792,856 describes a process for preparing carboxyalkyl cellulose esters. The process involves slurrying water wet carboxy(C1-C3 alkyl) cellulose (acid form) in a solvent, followed by treating the mixture with certain anhydrides in the presence of a strong acid catalyst. The mixture is heated to about 40° to 55° C. until the reaction is complete. Next, a mixture of water, an alkanoic acid, and optionally a C2-C5 alkanoic acid salt of an alkali or alkaline earth metal is added, in an amount that does not totally neutralize the strong acid catalyst. The solution is then heated to effect partial hydrolysis, and in an amount depending on the amount of strong acid catalyst, treated with a C2-C5 alkanoic acid salt of an alkali or alkaline earth metal dissolved in water and an alkanoic acid.
U.S. Pat. No. 4,590,265 relates to a method for the preparation of carboxylated cellulose esters, by reacting a cellulose ester with ozone at a temperature of about 25° to 80° C., for a period of time sufficient to yield the carboxylate cellulose ester product. The patent also discloses applications for the carboxylated cellulose esters, including metal coatings, pigment dispersions, wood coatings, and inks.
U.S. Pat. No. 4,520,192 describes a process for preparing carboxyalkyl acetyl celluloses with a carboxyalkyl degree of substitution (DS) per anhydroglucose unit of 0.2-2.5 and an acetyl DS of 0.5-2.8 and the metal salts of these materials. The preparation of the carboxyalkyl acetyl cellulose is accomplished by converting carboxyalkyl cellulose, sodium salt, into its acid form by soaking in an aqueous acid solution of sulfuric acid, hydrochloric acid, nitric acid, or acetic acid. This is followed by washing with water, solvent exchange of the water with acetic acid, and reaction with acetic anhydride in the presence of a catalyst (sulfuric acid, perchloric acid, sulfoacetic acid or zinc chloride) to give a trisubstituted carboxy acetyl cellulose, i e., carboxymethyl cellulose acetate, upon precipitation into water. Also described is a process for converting the carboxyalkyl acetyl cellulose product (acid form) to its corresponding sodium, potassium, calcium, or ammonium carboxylate salt. The acid forms of the trisubstituted carboxyalkyl acetyl celluloses were insoluble in water and aqueous lower chain alcohol solutions. These materials were soluble in acetone and methylene chloride/ethanol 9/1. The sodium salt of the carboxyalkyl acetyl cellulose was soluble in water, an aqueous lower alkyl alcohol containing a large amount of water, or aqueous acetone; however, it was insoluble in methylene chloride/ethanol 9/1. This process is believed to yield a product containing an unneutralized strong acid with high sulfur levels. Consequently, this ester would be unstable under general drying conditions or other heat treatment.
U.S. Pat. No. 3,435,027 describes a base catalyzed (sodium hydroxide) esterification of carboxymethyl cellulose (in the sodium salt form) in 70-85% acetone nonsolvent media with acetic, propionic, lactic, or stearyl anhydride. The procedure yields a product with a low degree of ester substitution and falls well short of a fully substituted cellulose.
USSR Patent 612933 describes a process for preparation of cellulose acetate ethers wherein an alkali activated cellulose (12% sodium hydroxide) is etherified with monochloroacetic acid followed by esterification with acetic anhydride in the presence of sulfuric acid catalyst. The process is limited to a low DS of the carboxymethyl substituent.
U.S. Pat. No. 3,789,117 discloses a process for preparing an enteric medicament coating from an organic solvent soluble cellulose derivative. The cellulose derivative's substitution has a carboxymethyl DS range of 0.3 to 1.2 in conjunction with at least one of the remaining hydroxyl groups being etherified or esterified. Ester groups include acetyl, propionyl, butyryl, nitric, or higher fatty acids. The degree of substitution is not specified.
RO 96929 (Romania) describes a carboxymethyl cellulose (CMC) acetate with a carboxymethyl degree of substitution of 0.5-3.0, acetyl DS of 0.4-2.9, and viscosity 150-1500 cP. This material was taught to be useful as an alkaline suspension stabilizer, solution thickener, and alkaline media binder. The material was prepared by mixing 70% acetic acid with CMC (Na salt), washing the acetic wet CMC with 50% acetic acid to remove the sodium acetate, and esterification of the acetic acid wet CMC-H with acetic anhydride for 1.5 hours at 50-110° C. in the presence of sulfuric acid.
U.S. Pat. No. 5,008,385 reports cellulose derivatives that can be synthesized by homogeneous reaction in dimethylacetamide and/or N-methylpyrrolidine containing LiCl. Included in their examples was CMC acetate (carboxymethyl DS 0.13-0.23, acetyl DS 2.54-2.15). These materials were tested for use in the production of fibers, filaments, or membranes.
Carbohydrate Research, 13, pp. 83-88, (1970) describes the preparation of CMC acetate by sulfuric acid catalyzed acetylation of CMC (carboxymethyl DS of 0.07) hydrolysis (acetylation and hydrolysis procedures taken from Maim, Ind. Eng. Chem., 38 (1946) 77), and evaluation of this material in membranes for reverse osmosis.
Holzforschung, 27(2), pp. 68-70, (1973) describes the rate of carboxymethylation and deacetylation of cellulose acetate in the presence of sodium hydroxide. This work showed that deacetylation and carboxymethylation occur simultaneously with the rate of deacetylation being faster than the rate of carboxymethylation. The highest carboxymethyl DS obtained was less than 0.1.
GB 2,284,421 discloses carboxymethyl cellulose alkanoates which are “lightly carboxymethylated”, i.e., having a degree of substitution per anhydroglucose unit of less than 0.2. Such polymers are taught to be useful as a coating agent for paper and papermaking or as a bonding agent for non-woven fabric, or can be extruded to form filaments or film, or can be used to produce shaped articles.
Despite these developments in preparing carboxylalkyl cellulose esters and using them in coating applications, there remains a need in the art for a precoat or size that decreases fiber raising, and increases holdout, particularly when applied as an aqueous precoat or size composition to a substrate. Such a process should preferably reduce the number of coatings required to achieve a desired result, and produce a substrate that is compatible with an overcoat. This invention meets this need.